1. Technical Field
The present disclosure relates to a microelectromechanical sensing structure of MEMS (microelectromechanical system) type for a capacitive acoustic transducer, in particular, for a microelectromechanical capacitive microphone, to which the ensuing treatment will make explicit reference, without this implying any loss of generality. The microelectromechanical sensing structure includes an element for limiting the oscillations of a membrane. Furthermore, the present disclosure relates to a method for manufacturing the microelectromechanical sensing structure.
2. Description of the Related Art
As is known, an acoustic transducer, for example a MEMS microphone of a capacitive type, generally comprises a microelectromechanical sensing structure designed to transduce acoustic waves, i.e., pressure waves, into an electrical quantity, in particular into an electrical quantity indicating a capacitive variation. In addition, the MEMS microphone comprises read electronics, which is designed to carry out appropriate operations of processing (for example, operations of amplification and filtering) of the electrical quantity, so as to supply an electrical output signal, typically formed by a voltage.
The microelectromechanical sensing structure usually comprises a mobile electrode, provided as diaphragm or membrane, which is arranged facing a fixed electrode so as to form, together with this fixed electrode, the plates of a sensing capacitor, which is of the variable-capacitance type. The mobile electrode is generally anchored, by means of a perimetral portion thereof, to a substrate, whereas a central portion thereof is free to bend following upon incidence of a pressure wave. The variation in capacitance of the sensing capacitor is caused by the deflection of the membrane that forms the mobile electrode, this membrane being precisely put in oscillation by the pressure wave.
By way of example, FIG. 1 shows a microelectromechanical sensing structure 1 of a capacitive microphone M, as described for example in the patent No. EP2252077, also published as U.S. Patent Publication No. 2010/284553, filed in the name of the present applicant.
The microelectromechanical sensing structure 1 comprises a membrane 2, which is mobile, is made of conductive material, and is arranged facing a rigid plate 3, generally known as “back plate,” which is, as has been said, rigid, at least if compared with the membrane 2, which is, instead, flexible.
The rigid plate 3 is formed by a first plate layer 4a, made of conductive material and set facing the membrane 2, and by a second plate layer 4b, made of insulating material.
The first plate layer 4a forms, together with the membrane 2, a sensing capacitor.
The second plate layer 4b is arranged on the first plate layer 4a, except for portions in which it extends through the first plate layer 4a so as to form protuberances P of the rigid plate 3, which extend towards the underlying membrane 2 and have the function of preventing adhesion of the membrane 2 to the rigid plate 3, as well as that of limiting the oscillations of the membrane 2.
The microelectromechanical sensing structure 1 further comprises a substrate 5 made of semiconductor material and an insulation layer 9, which is made, for example, of silicon nitride (SiN) and is arranged on top of the substrate 5, with which it is in direct contact.
In detail, the membrane 2 may have a thickness comprised, for example, in the range 0.3-1.5 μm and may be made of polysilicon, while the first and second plate layers 4a, 4b may have thicknesses, respectively, comprised, for example, in the ranges 0.5-2 μm and 0.7-2 μm and may be made, respectively, of polysilicon and silicon nitride. For example, the first and second plate layers 4a, 4b may have thicknesses, respectively, of 0.9 μm and 1.2 μm.
In use, the membrane 2 undergoes deformation as a function of the incident pressure wave. Furthermore, the membrane 2 is suspended over the substrate 5 and the insulation layer 9 and gives out directly onto a first cavity 6a. The first cavity 6a is formed within the substrate 5, by etching a back surface Sb of the substrate 5, which is opposite to a front surface Sa of the same substrate 5, on which the insulation layer 9 rests; this front surface Sa is arranged in the proximity of the membrane 2. Furthermore, the etching is of a through type, i.e., it is made in such a way that, at the end of the etching process, the first cavity 6a defines a through hole that extends between the front surface Sa and the back surface Sb.
More in particular, the first cavity 6a is formed by a first portion 7a and a second portion 7b, which communicate with one another. Arranged between the first and second portions 7a, 7b of the first cavity 6a is a perforated diaphragm X, formed by top portions (not etched) of the substrate 5 overlaid by portions of the insulation layer 9 that overlie these top portions.
The perforated diaphragm X forms an opening TH, the longitudinal axis H of which is normal to the membrane 2 when the latter is in the resting condition. Furthermore, the opening TH sets the first and second portions 7a, 7b of the first cavity 6a in communication; consequently, the opening TH defines a sort of third portion of the first cavity 6a. 
The opening TH has, in a direction perpendicular to the longitudinal axis H and parallel to the front surface Sa, a cross section that is entirely overlaid by the membrane 2, which moreover extends laterally up to overlying at least part of the perforated diaphragm X. In other words, the membrane 2, which is vertically aligned with the perforated diaphragm X, has an area, in top plan view, larger than the area of the opening TH. Consequently, a central portion of the membrane 2 overlies the opening TH, while peripheral portions of the membrane 2 overlie corresponding portions of the substrate 5, as well as the portions of the insulation layer 9 arranged on top of the latter.
More in particular, as may be seen in FIG. 2, each between the opening TH and the first portion 7a of the first cavity 6a defines a parallelepipedal shape; hence it has, in a plane perpendicular to the longitudinal axis H, the shape of a square or of a rectangle. Consequently, the cross section of the opening TH in a plane perpendicular to the longitudinal axis H has a rectangular or square shape, and the opening TH itself is delimited by four walls parallel to the longitudinal axis H, formed by the perforated diaphragm X. In this connection, illustrated in FIG. 1 are a first opening wall W1 and a second opening wall W2, opposite to one another, whereas illustrated in FIG. 2 are the first opening wall W1 and a third opening wall W3. Furthermore, even though it is not illustrated in FIG. 2, also the second portion 7b of the first cavity 6a substantially defines a parallelepipedal shape.
Even more in particular, the opening TH and the first and second portions 7a, 7b of the first cavity 6a are aligned along the longitudinal axis H. The perforated diaphragm X extends with a width l (measured in a direction perpendicular to the longitudinal axis H) towards the inside of the microelectromechanical sensing structure 1, starting from an internal surface Sin that delimits the first portion 7a of the first cavity 6a. The internal surface Sin is formed by four inner walls, and the perforated diaphragm X departs from said four inner walls. Illustrated in FIG. 1 are a first inner wall L1 and a second inner wall L2, opposite to one another.
The first cavity 6a is also known, as a whole, as “back chamber,” in the case where the pressure wave impinges first on the rigid plate 3, and then on the membrane 2. In this case, the first cavity 6a performs the function of reference pressure chamber. Alternatively, it is in any case possible for the pressure waves to reach the membrane 2 through the first cavity 6a, which in this case performs the function of acoustic access port, and is hence known as “front chamber.” In what follows, however, reference is made, except where otherwise specified, to the case where the first cavity 6a functions as back chamber, and the front chamber is formed by a second cavity 6b, which is delimited at the top and at the bottom, respectively, by the first plate layer 4a and by the membrane 2.
In greater detail, the membrane 2 has a first surface F1 and a second surface F2, which are opposite to one another and face, respectively, the first cavity 6a and the second cavity 6b. The first and second surfaces F1, F2 are hence in fluid communication with the back chamber and the front chamber, respectively; i.e., they are in contact with the fluids contained therein.
The membrane 2 is moreover anchored to the substrate 5 by means of membrane anchorages 8, which are formed by protuberances of the membrane 2, which extend, starting from peripheral regions of the membrane 2, towards the substrate 5. The insulation layer 9 enables, amongst other things, electrical insulation of the membrane anchorages 8 from the substrate 5.
The membrane anchorages 8 perform the function of suspending the membrane 2 over the substrate 5 at a certain distance therefrom; the value of this distance is a function of a compromise between the linearity of response at low frequencies and the noise of the capacitive microphone M.
In order to enable relief of the residual (tensile and/or compressive) stresses in the membrane 2, for example deriving from the manufacturing method, through openings 10 are formed through the membrane 2, in particular in the proximity of each membrane anchorage 8. The through openings 10 enable “equalization” of the static pressure present on the first and second surfaces F1, F2.
The rigid plate 3 is anchored to the substrate 5 by means of plate anchorages 11, which are connected to peripheral regions of the rigid plate 3. The plate anchorages 11 are formed by pillars made of the same conductive material as the first plate layer 4a, these pillars being arranged on top of the substrate 5 and being electrically insulated from the substrate 5 by means of the insulation layer 9. Furthermore, these pillars form a single piece with the first plate layer 4a. 
The microelectromechanical sensing structure 1 further comprises a first sacrificial layer 12a, a second sacrificial layer 12b, and a third sacrificial layer 12c, arranged on top of one another. In particular, the third sacrificial layer 12c is arranged on top of the second sacrificial layer 12b, which is in turn arranged on top of the first sacrificial layer 12a, the latter being set on top of, and in direct contact with, the insulation layer 9.
The first, second, and third sacrificial layers 12a-12c are arranged, in top plan view, on the outside of the area occupied by the membrane 2 and by the plate anchorages 11. Peripheral portions of the rigid plate 3 extend on top of top portions of the third sacrificial layer 12c. 
The rigid plate 3 moreover has a plurality of holes 13, which extend through the first and second plate layers 4a, 4b, have preferably circular sections and perform the function of favoring, during the manufacturing steps, removal of the underlying sacrificial layers. Furthermore, in use, the holes 13 enable free circulation of air between the rigid plate 3 and the membrane 2, in effect rendering the rigid plate 3 acoustically transparent. The holes 13 hence function as acoustic access port to enable the pressure waves to reach and deform the membrane 2.
The microelectromechanical sensing structure 1 further comprises an electrical membrane contact 14 and an electrical rigid-plate contact 15, which are made of conductive material and are used, in use, for biasing the membrane 2 and the rigid plate 3, as well as for picking up a signal indicating the capacitive variation consequent upon deformation of the membrane 2 caused by the pressure waves.
As illustrated in FIG. 1, the electrical membrane contact 14 extends partly into the second plate layer 4b. Furthermore, the electrical membrane contact 14 is electrically insulated from the first plate layer 4a and is electrically connected to the membrane 2, via to a conductive path (not illustrated).
The electrical rigid-plate contact 15 extends through the second plate layer 4b until it contacts the first plate layer 4a. 
In a known way, the sensitivity of the capacitive microphone M depends upon the mechanical characteristics of the membrane 2, as well as upon the assembly of the membrane 2 and of the rigid plate 3. Furthermore, the performance of the capacitive microphone M depends upon the volume of the back chamber and upon the volume of the front chamber. In particular, the volume of the front chamber determines the upper resonance frequency of the capacitive microphone M, and hence its performance at high frequencies. In general, in fact, the smaller the volume of the front chamber, the greater the upper cutoff frequency of the capacitive microphone M. Furthermore, a large volume of the back chamber enables improvement of the frequency response and sensitivity of the capacitive microphone M itself.
Given this, as mentioned previously, in use the membrane 2 oscillates alternately in the direction of the rigid plate 3 or in the direction of the substrate 5. The membrane 2 hence moves alternately in the direction of the first plate layer 4a or in that of the substrate 5. In particular, each point of the membrane 2 moves in a substantially sinusoidal way, in a direction perpendicular to a direction defined by the membrane 2 itself in resting conditions. Again, more in particular, each point of the membrane 2 moves between a corresponding top position, which is at a distance from the position assumed by the same point in the resting condition equal to a corresponding displacement upwards, and a corresponding bottom position, which is at a distance from the position assumed by the same point in the resting condition equal to a corresponding displacement downwards. If we designate by “amplitude” the sum of the displacement downwards and of the displacement upwards, this amplitude is greater for the points of the membrane 2 that are further away from the membrane anchorages 8, also referred to as central points of the membrane 2, whereas it is smaller (at the limit, zero) for the points of the membrane 2 closer to the membrane anchorages 8, also referred to as peripheral points of the membrane 2. In what follows, reference will be made to the maximum displacement upwards and to the maximum displacement downwards to indicate the corresponding displacements of the point of the membrane 2 that oscillates with the maximum amplitude. Furthermore, in general, reference will be made to the amplitude of the oscillations to indicate the amplitude of oscillation of the point of the membrane 2, among all the points of the membrane 2, that oscillates with maximum amplitude.
Since the degree of the oscillations of the membrane 2 may be such as to cause mechanical failure of the membrane 2 itself, solutions designed to limit the amplitude of the oscillations have been proposed.
For example, as mentioned previously, the protuberances P themselves of the rigid plate 3 function as upper stopper for the oscillations of the membrane 2. In fact, in the presence of large oscillations, the central portion of the membrane 2 abuts against one or more of these protuberances P, with consequent limitation of the amplitude of the oscillation. The protuberances P hence enable limitation of the maximum displacement upwards to which the membrane 2 is subjected.
As regards the maximum displacement downwards, its limitation is generally achieved by means of an appropriate sizing of the perforated diaphragm X. In fact, since the membrane 2 at least partially overlaps the perforated diaphragm X, in the presence of considerable oscillations, parts of the membrane 2 come to abut against the perforated diaphragm X, which to some extent limits deformation of the membrane 2. In other words, the membrane 2 is not free to undergo deformation, in the presence of pressure waves of sufficiently large amplitude within the opening TH, without contacting the perforated diaphragm X. The perforated diaphragm X hence functions as lower stopper for the oscillations of the membrane 2, since it prevents the central portion of the membrane 2 from oscillating with the same amplitude that would be obtained in the case of absence of the perforated diaphragm X.
However, the aforementioned mechanism of limitation of the maximum displacement downwards has proven satisfactory only in the case of oscillations of small amplitude. In fact, in the case of oscillations of considerable amplitude, it is possible for the membrane 2 to be subject in any case to failure, notwithstanding the stopping action exerted by the perforated diaphragm X.
There is consequently felt in the sector the need to provide a microelectromechanical sensing structure for a capacitive acoustic transducer that will solve at least in part the drawbacks of the known art.